Films or sheets of linear polyester have been commercially available for many years. The film has excellent draw orientation and has proved to be especially well suited for the biaxial film orientation process resulting in films of outstanding properties. The film is also known as polyethylene terephthalate or PET and is strong and has excellent inherent chemical and thermal stability properties. The polymer preparation and film manufacturing processes are well known to those skilled in the art and are recited in many texts, including the Encyclopedia of Polymer Science and Engineering, second edition, volume 12, by John Wiley and Sons, Inc., pages 1 through 313; and numerous U.S. and foreign patents.
The myriad uses of polyester film have resulted in the need for modification of the basic film to provide special properties such as improved slip and improved release properties. The control of surface slip is one of the prime requirements for the commercial use of polyester film. Slip is critical to the processability of the film, especially thin film. In the past, slip has been controlled by incorporating organic and inorganic filters to enhance surface roughness. However, the addition of these additives has caused increased haze in the film. These additives include inert particles such as silicas, china clay, aluminum silicates, calcium phosphates and glass particles. The addition of these fillers improves the winding and slitting properties of the polyester film, however, their presence in the film produces large surface asperities which makes polyester film unsuitable for demanding applications such as when the film is metallized with aluminum or gold and an absolutely smooth metallized surface is required. Examples of metallized films requiring a smooth surface are polyester films used for solar window applications or certain films demanding certain aesthetic properties.
U.S. Pat. No. 4,274,025 teaches a linear polyester film containing particles of inorganic material that has an improved resistance to fibrillation, delamination, and tearing. Included is a listing of the inorganic materials synthetic silica, calcium borate, calcium carbonate, magnesium carbonate, barium sulphate, calcium or aluminum silicate and glass beads. The particle size for the inorganic material is between 2 to 10 microns, but the amount of additive is stated to range between 2000 to 9000 parts per million (ppm) based upon the weight of the film. The patent teaches that adding less than 2000 ppm does not render the film surface sufficiently rough to promote a satisfactory sliding action for good slip or frictional purposes. Quite surprisingly, the linear polyester film of the present invention has excellent slip properties and it contains glass spheres in an amount 1000 times less than taught for U.S. Pat. No. 4,274,025.
U.S. Pat. No. 4,375,494 teaches a polyester film composite of a highly crystalline, molecularly oriented first layer of linear polyester, a heat sealable second layer, adherent to the first layer, of an amorphous linear polyester. The second layer contains from 50 to 2000 ppm (based upon the weight of the second layer) of a finely-divided particulate additive having an average particle size in the range of 2 to 10 microns together with 1000 to 10,000 ppm (based upon the weight of the second film) of smaller, finely divided particles having an average particle in the range of 0.005 to 1.8 microns. Both particulate additives are substantially uniformly dispersed throughout the second layer. The particulate additive is stated to be natural or synthetic silica, glass beads, calcium borate, calcium carbonate, magnesium carbonate, barium sulphate, calcium silicate, calcium phosphate, aluminum trihydride, aluminum silicates and titanium oxide or certain polymeric materials. Ideally, the particles are substantially spherical in shape. The addition of the additives to the second layer results in a film having improved handling, heat sealing properties, and anti-blocking properties.
The amount of additives used in the film composite of this patent are much greater than the amount of additive material that has been found to be useful in the practice of the present invention.